I feel that off-topic questions usually shouldn't be reaped and usually shouldn't be approved for their section. But replying to them is often appropriate.
Not approving a question prevents it from showing in that section. If it isn't on-topic for that section, then it shouldn't show. This doesn't stop it from showing up via searching or Super Search. I don't think off-topic material should be maximally accessible anyway. q-:
So I agree with tilly that replying shouldn't imply approving (with a note that AnonyMonk questions, unfortunately, should be approved with more leniency).
(but my friends call me "Tye")